Biological characteristics of type C viruses isolated from different Friend erythroleukaemic cells

Chromatin reorganization during emergence of malignant Friend tumors: early changes in H2A and H2B variants and nucleosome repeat length

Serial examination of five newly derived Friend murine tumors during early subcutaneous passages showed continuing changes in chromatin composition and structure over the first 10 to 20 passages followed by a period of stabilization over the subsequent 20 passages. These changes were reflected in a decrease in two major histone variants, H2A.1 and H2B.2, with a coordinate increase in histone variants, H2A.2 and H2B.1, and a changing nucleosome repeat length (NRL). The absolute values differed among the five tumors, but all five showed the same general direction of change. There was no obvious relationship among the NRL, H2A, and H2B histone variant values. A low H2A.1/H2A.2 ratio was found in Friend tumors of high malignant potential. Cell lines derived in vitro also showed directional changes in the H2A and H2B variants similar to those of their tumor cell parents, but with different kinetics. Our findings suggest that Friend tumor establishment is accompanied by an early period of chromatin reorganization marked by changes in several parameters of chromatin structure.

Ca2+, Mg2+-dependent endonuclease activity in different subpopulations of spleen cells from normal and erythroleukemic mice

We have detected Ca2+, Mg2+-dependent endonuclease activity in spleen cells of normal, Friend erythroleukemic, and phenylhydrazine-treated mice. When nuclei were isolated and incubated in the presence of Ca2+ and Mg2+ ions, the activity resulted in the production of 3'-OH termini in the cellular DNA and the release of chromatin due to internucleosomal DNA fragmentation. This enzyme activity was chromatin-bound and could be extracted from chromatin in an active form in 0.35 M KCl. The majority of endonuclease activity from erythroleukemic spleens was present in nuclei of precursor erythroid cells of low buoyant density (1.025-1.05 g/ml). Uninfected normal splenic tissue contained an endonuclease activity which was almost entirely confined to a B-lymphocyte population of high buoyant density (greater than 1.07 g/ml). Erythroid cell-enriched spleens from phenylhydrazine-treated mice exhibited a distribution of endonuclease activity in cells at low and high densities reflecting a mixture of erythroid and lymphoid cells. Cloned erythroleukemic cell lines propagated in vitro lacked cells of low density and showed no detectable endonuclease activity. However, nuclei from these cell lines were susceptible to exogenously added endonuclease extracted from erythroleukemic spleen cells. These same cell lines propagated as subcutaneous tumors contained endonuclease activity and a morphologically-similar low-density cell population which accounted for the endonuclease activity in these tumors. Nuclei from cloned lymphoid cell lines, representing different B-lymphocyte phenotypes, showed differences in the presence of endonuclease activity. Among the cell lines tested, only those expressing late B-cell markers showed detectable endonuclease activity.

Regulation of mRNA utilization in mouse erythroleukemia cells induced to differentiate by exposure to dimethyl sulfoxide

Mouse erythroleukemia cells contain several abundant mRNA species that occur to a considerable extent as untranslated molecules. For two of these species, which code for polypeptides P40 and P21, the proportion of molecules engaged in translation decreases rapidly after exposure of the cells to dimethyl sulfoxide. The extent of utilization of a third species, the P36 mRNA, is not altered. The rate of production of the P40 mRNA does not appear to be affected in the dimethyl sulfoxide-treated cells. The P21 mRNA appears to be produced in increasing amounts, leading to a large accumulation of untranslated molecules in the cytoplasm. The mRNA for actin remains nearly fully utilized during this process, but its intracellular concentration decreases, thus resulting in a reduction in the amounts present in polysomes. The results indicate that some mRNA species in mouse tumor cells are subject to a translational repression process that can serve to regulate selectively the extent of expression of the corresponding genes.

Regulation of mRNA utilization in mouse erythroleukemia cells induced to differentiate by exposure to dimethyl sulfoxide

Mouse erythroleukemia cells contain several abundant mRNA species that occur to a considerable extent as untranslated molecules. For two of these species, which code for polypeptides P40 and P21, the proportion of molecules engaged in translation decreases rapidly after exposure of the cells to dimethyl sulfoxide. The extent of utilization of a third species, the P36 mRNA, is not altered. The rate of production of the P40 mRNA does not appear to be affected in the dimethyl sulfoxide-treated cells. The P21 mRNA appears to be produced in increasing amounts, leading to a large accumulation of untranslated molecules in the cytoplasm. The mRNA for actin remains nearly fully utilized during this process, but its intracellular concentration decreases, thus resulting in a reduction in the amounts present in polysomes. The results indicate that some mRNA species in mouse tumor cells are subject to a translational repression process that can serve to regulate selectively the extent of expression of the corresponding genes.